An antiferroelectric liquid crystal panel stabilizes into an antiferroelectric state when the liquid crystal panel is left in a condition of no voltage application (zero volts). This stable condition is hereinafter referred to as the neutral state. The antiferroelectric liquid crystal panel can be constructed to produce a dark display in the neutral state or a bright display in the neutral state. The present invention is applicable to both modes of operation. The description given herein deals with a panel that produces a dark display in the neutral state. It should also be noted that the antiferroelectric liquid crystal panels used in our investigations and embodiments have been treated by isotropic processing in which the panel is heated in a furnace or the like and then cooled to its normal operating temperature. This treatment is applied not only to antiferroelectric liquid crystal panels but to other conventional liquid crystal panels, as necessary, in order to stabilize the condition of liquid crystal layers; if the liquid crystal condition is stable from the beginning, this treatment is not particularly needed. Further, even when this treatment is needed, the treatment need only be performed once in the final step of the panel manufacturing process. Therefore, whether to perform or not perform this treatment can be determined freely.
FIG. 1 is a diagram showing, as an example, the optical transmittance of an antiferroelectric liquid crystal as a function of applied voltage with the applied voltage plotted along the abscissa and the optical transmittance plotted along the ordinate.
When an increasing positive voltage is applied to the liquid crystal which is in the neutral state at point 0, the optical transmittance begins to increase abruptly at voltage Ft and reaches approximately the maximum transmittance at voltage Fs to enter a saturated ferroelectric state. After that, if the applied voltage is further increased, the optical transmittance remains substantially unchanged. Next, when the applied voltage is gradually decreased, the optical transmittance begins to drop abruptly at voltage At and reaches almost zero at voltage As to return to the antiferroelectric state. Likewise, when the applied voltage is increased from 0 V in the negative direction, the optical transmittance begins to increase abruptly at voltage -Ft and reaches approximately the maximum transmittance at voltage -Fs to enter a saturated ferroelectric state. After that, when the applied voltage is gradually brought toward 0 V, the optical transmittance begins to drop abruptly at voltage -At and reaches almost zero at voltage -As to return to the antiferroelectric state. In this way, the ferroelectric state of the liquid crystal can be achieved by applying either a positive voltage or a negative voltage. The former case will be referred to as the (+) ferroelectric state and the latter case as the (-) ferroelectric state. Further, .vertline.Ft.vertline. will be referred to as the ferroelectric threshold voltage, .vertline.Fs.vertline. as the ferroelectric saturation voltage, .vertline.At.vertline. as the antiferroelectric threshold voltage, and .vertline.As.vertline. as the antiferroelectric saturation voltage.
Generally, a matrix-addressed liquid crystal panel comprises N row electrodes and M column electrodes arranged in a matrix form. To drive the panel, a scan signal is applied to each row electrode via a row electrode driving circuit, and a display signal, which is dependent on the display data of each pixel (though the signal may contain a portion that does not depend on the display data), is applied to each column electrode via a column electrode driving circuit, thereby applying to the liquid crystal layer a voltage representing the difference between the scan signal and the display signal (the difference voltage will be hereinafter simply referred to as the "synthetic voltage"). The period required to scan all the row electrodes (one vertical scan period) is usually known as one frame (or one field). In liquid crystal driving, the polarity of the drive voltage is reversed for each frame (or for every multiple frames) to prevent an ill effect on the liquid crystal (for example, deterioration due to clustering of ions in a particular direction).
When the scan signal applied to one row electrode is examined, its vertical scan period consists of N horizontal scan periods (in some cases, an additional period may be included). The horizontal scan period during which a scan voltage for determining the display state of the pixels in the active row (hereinafter referred to as the "selection voltage") is applied is called the selection period tw for that row, and the other horizontal scan periods are collectively called the non-selection periods.
Usually, in an antiferroelectric liquid crystal panel, when applying the selection voltage, it is determined whether the liquid crystal in the antiferroelectric state should be maintained in that state or be caused to make a transition to the ferroelectric state. For this purpose, a period during which the liquid crystal state is set in the antiferroelectric state is required prior to the application of the selection voltage; hereinafter, this period is called the relaxation period ts. During other periods than the selection period tw and relaxation period ts, the liquid crystal must be held in the determined state; this period is called the holding period tk.
FIG. 2 is a diagram showing the scan signal waveform (Pa), display signal waveform (Pb, Pb'), and composed voltage waveform (Pc, Pc') applied to an arbitrary attention pixel in an antiferroelectric liquid crystal panel in accordance with the drive method illustrated in FIGS. 1 and 2 in Japanese Patent Unexamined Publication NO. 4-362990, along with light transmittance L100, L0.
In FIG. 2, F1 and F2 denote a first frame and a second frame, respectively. The figure shows the case where the polarity of the drive voltage is reversed for each frame. As can be seen from the figure, the polarity of the drive voltage is simply reversed between the first frame F1 and the second frame F2, and as is apparent from FIG. 1, the liquid crystal operation is symmetrical relative to the polarity of the drive voltage. The following description, therefore, deals only with the first frame, unless otherwise noted.
In FIG. 2, one frame is divided into three periods: the selection period tw, the holding period tk, and the relaxation period ts. The selection period tw is further divided into periods tw1 and tw2 of equal length. The voltage of the scan signal Pa in the first frame F1 is set as shown below. Of course, the polarity of the voltage is reversed in the second frame F2. Here, .+-.V1 is the selection voltage.
Period tw1 tw2 tk ts
Scan signal voltage 0+V1+V3 0
The display signal is set as shown below according to the display state of the attention pixel. Note that the voltages indicated by the symbol * depend on the display data of other pixels in the same column.
Period tw1 tw2 tk ts
ON display signal voltage +V2 -V2 * *
OFF display signal voltage +V2 -V2 * *
In the hysteresis curves shown in FIG. 1, the curve, for example, from As to Ft or from At to Fs, is generally not flat; therefore, if the voltage applied to the liquid crystal during the holding period tk is held in one particular direction depending on the display signal, variation is caused in the brightness during that period. To avoid this, the polarity of the display signal is usually reversed so that its average value becomes zero over one horizontal scan period. More specifically, the polarity of the display signal is reversed between the period twl and the period tw2.
In FIG. 2, Pb, Pc, and L100 indicate the display signal waveform, the synthetic voltage waveform, and the optical transmittance, respectively, when all the pixels in the column containing the attention pixel are in the ON (bright) state. In this case, if the voltage (synthetic voltage) applied to the liquid crystal during the period tw2 is .vertline.V1+V2.vertline.&gt;.vertline.Ft.vertline. (see FIG. 1), the liquid crystal begins to make a transition to the ferroelectric state, and the optical transmittance increases. In the holding period tk, if .vertline.V3-V2.vertline.&gt;.vertline.At.vertline., the bright state can be maintained. In the relaxation period ts, if .vertline.V2.vertline.&lt;.vertline.As.vertline., the optical transmittance decreases with time, and the liquid crystal relaxes from the ferroelectric state back to the stable antiferroelectric state.
In FIG. 2, Pb', Pc', and L0 indicate the display signal waveform, the synthetic voltage waveform, and the optical transmittance, respectively, when all the pixels in the column containing the attention pixel are in the OFF (dark) state. In this case, the dark state can be produced if the composed voltage in the period tw2 is .vertline.V1-V2.vertline.&lt;.vertline.Ft.vertline., the voltage applied during the holding period tk is .vertline.V3+V2.vertline.&lt;.vertline.Ft.vertline., and the voltage applied during the relaxation period ts is .vertline.V2.vertline.&lt;.vertline.Ft.vertline..